Currently, various display devices are used in a variety of applications, including liquid crystal display devices. In commonly-used display devices, each pixel is composed of three subpixels for displaying three primaries of light, i.e., red, green and blue, whereby multicolor display is achieved.
However, conventional display devices have a problem in that they can only display colors in a narrow range (referred to as a “color gamut”). FIG. 16 shows a color gamut of a conventional display device which performs display by using three primaries. FIG. 16 is an xy chromaticity diagram in an XYZ color system, where a color gamut is shown by a triangle whose apices are at three points corresponding to the three primaries of red, green and blue. Also shown in the figure are plotted colors (represented by “×” symbols) of various objects existing in nature, as taught by Pointer (see Non-Patent Document 1). As can be seen from FIG. 16, there are some object colors which do not fall within the color gamut. Thus, display devices which perform display by using three primaries are unable to display some object colors.
Therefore, in order to broaden the color gamut of a display device, there has been proposed a technique which increases the number of primary colors to be used for displaying to four or more.
For example, as shown in FIG. 17, Patent Document 1 discloses a liquid crystal display device 800 each of whose pixels P is composed of six subpixels R, G, B, Ye, C and M for displaying red, green, blue, yellow, cyan, and magenta. The color gamut of the liquid crystal display device 800 is shown in FIG. 18. As shown in FIG. 18, a color gamut which is represented as a hexagonal shape whose apices are at six points corresponding to the six primary colors substantially encompasses all object colors. Thus, the color gamut can be broadened by increasing the number of primary colors to be used for displaying. In the present specification, display devices which perform display by using four or more primary colors will be collectively referred to as “multiprimary display devices”, and liquid crystal display devices which perform display by using four or more primary colors will be referred to as “multiprimary liquid crystal display devices (or simply, multiprimary LCDs)”. Moreover, conventional commonly-used display devices which perform display by using three primaries will be collectively referred to as “three-primary display devices”, and liquid crystal display devices which perform display by using three primaries will be referred to as “three-primary liquid crystal display devices (or simply, three-primary LCDs)”.
As the formats of a video signal to be input to a three-primary display device, the RGB format, the YCrCb format, and the like are commonly used. A video signal of these formats contains three parameters (thus being a three-dimensional signal, as it were), thus allowing the luminances of the three primaries (red, green, and blue) used for displaying to be uniquely determined.
In order to perform display with a multiprimary display device, it is necessary to convert a video signal of a format for three-primary display devices to a video signal containing more parameters (four or more parameters). Such a video signal corresponding to four or more primary colors will be referred to as a “multiprimary signal” in the present specification.    [Patent Document 1] Japanese National Phase PCT Laid-Open Publication No. 2004-529396    [Non-Patent Document 1] M. R. Pointer, “The gamut of real surface colors,” Color Research and Application, Vol. 5, No. 3, pp. 145-155 (1980)